An SCR system is known, which is provided with a selective catalytic reduction NOx catalyst (hereinafter referred to as “SCR catalyst” as well), wherein NOx, which is contained in the exhaust gas of an internal combustion engine, is purified by using ammonia as a reducing agent. An addition valve, which supplies an aqueous urea solution (urea-water solution) into the exhaust gas, is installed in some cases on the upstream side from the SCR catalyst. The aqueous urea solution, which is added into the exhaust gas, is hydrolyzed by the heat of the exhaust gas and the heat of the SCR catalyst, and the aqueous urea solution is converted into ammonia which is adsorbed by the SCR catalyst. The aqueous urea solution is stored, for example, in a tank, and the aqueous urea solution is finite. Therefore, it is necessary to replenish the tank with the aqueous urea solution, for example, by a user. If the aqueous urea solution is used up without performing the replenishment with the aqueous urea solution, the reducing agent cannot be supplied to the SCR catalyst. Therefore, it is feared that NOx may be released into the atmospheric air.
In this context, a technique is known, in which a warning is given if the remaining amount of the aqueous urea solution is decreased, and the output of an internal combustion engine is thereafter restricted in a stepwise manner in response to the remaining amount of the aqueous urea solution (see, for example, Patent Literature 1). Further, the following technique is known. That is, an SCR catalyst is provided, and a storage reduction NOx catalyst (NOx storage reduction catalyst) (hereinafter referred to as “NSR catalyst” as well), which occludes NOx contained in the exhaust gas and which reduces NOx if the air-fuel ratio of the exhaust gas is not more than the theoretical air-fuel ratio, is provided, wherein the NSR catalyst is principally used if the remaining amount of the aqueous urea solution is decreased (see, for example, Patent Literature 5).